The present invention relates generally to spectrometers used to determine the spectral characteristics of a source of radiant energy and specifically, to a device to measure efficiently the spectral width of a spectrally narrow source.
High resolution measurements of the spectral width of a nearly monochromatic source of radiant energy are most commonly made with a scanning Fabry-Perot interferometer. The Fabry-Perot acts as a tunable filter, transmitting the energy from the source that is in the passband of the interferometer at any scan position (frequency band). If the interferometer has a passband that is significantly narrower than the linewidth of the source, scanning the interferometer will measure the spectrum of the source (radiant power vs. wavelength or frequency).
An alternative approach to the scanning filter technique is to use heterodyne spectroscopy in which a monochromatic local oscillator that is spectrally narrower than the source to be measured is mixed with radiation from the signal source. The resulting beat frequency is then measured to determine the spectrum of the unknown source.
Instead of a scanning filter operating in frequency space, Michelson used the two-beam interferometer bearing his name and examined the interference contrast (variation in intensity vs. optical delay) as a function of scan distance to infer the linewidth (or sometimes existence of a spectral doublet) of a radiant source. Scanning Michelson interferometers are now highly developed and available for measuring the spectrum of a complex radiant source by means of Fourier transform spectroscopy. A Fourier transform is used to convert the interferogram (radiant power vs. optical path difference between the two arms of the interferometer) to a spectrum.
One disadvantage of these scanning interferometers is that, during the scanning period, the source could fluctuate and this would show up in the spectral width measurement, degrading its accuracy. Additionally, the mechanical hardware required in a scanning interferometer is extremely complex and must be very precise in its movement of the mirrors in order to prevent changes in mirror inclination while permitting changes in the optical path length.
A further disadvantage of the prior art scanning interferometer (especially the Fabry-Perot interferometer) is the relatively small amount of radiant power which is transmitted by the interferometer onto the detector. Because the Fabry-Perot interferometer acts as a spectral filter, only the radiant energy in the narrow spectral interval is transmitted to the detector.